Method and system for integrated power combiners

ABSTRACT

A system for integrated power combiners is disclosed and may include receiving optical signals in input optical waveguides and phase-modulating the signals to configure a phase offset between signals received at a first optical coupler, where the first optical coupler may generate output signals having substantially equal optical powers. Output signals of the first optical coupler may be phase-modulated to configure a phase offset between signals received at a second optical coupler, which may generate an output signal having an optical power of essentially zero and a second output signal having a maximized optical power. Optical signals received by the input optical waveguides may be generated utilizing a polarization-splitting grating coupler to enable polarization-insensitive combining of optical signals. Optical power may be monitored using optical detectors. The monitoring of optical power may be used to determine a desired phase offset between the signals received at the first optical coupler.

CROSS-REFERENCE TO RELATED APPLICATIONS/INCORPORATION BY REFERENCE

This application is a continuation of application Ser. No. 15/812,517filed on Nov. 14, 2017, which is a continuation of application Ser. No.15/231,356 filed on Aug. 8, 2016, now U.S. Pat. No. 9,823,495, which isa continuation of application Ser. No. 14/149,626 filed on Jan. 7, 2014,now U.S. Pat. No. 9,417,466, which is a divisional application ofapplication Ser. No. 13/157,642 filed on Jun. 10, 2011, now U.S. Pat.No. 8,625,935, which makes reference to and claims priority to U.S.Provisional Application Ser. No. 61/397,738 filed on Jun. 15, 2010. Eachof the above stated applications is hereby incorporated herein byreference in its entirety.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[Not Applicable]

MICROFICHE/COPYRIGHT REFERENCE

[Not Applicable]

FIELD OF THE INVENTION

Certain embodiments of the invention relate to signal processing. Morespecifically, certain embodiments of the invention relate to a methodand system for integrated power combiners.

BACKGROUND OF THE INVENTION

As data networks scale to meet ever-increasing bandwidth requirements,the shortcomings of copper data channels are becoming apparent. Signalattenuation and crosstalk due to radiated electromagnetic energy are themain impediments encountered by designers of such systems. They can bemitigated to some extent with equalization, coding, and shielding, butthese techniques require considerable power, complexity, and cable bulkpenalties while offering only modest improvements in reach and verylimited scalability. Free of such channel limitations, opticalcommunication has been recognized as the successor to copper links.

Optical communication systems have been widely adopted for applicationsranging from internet backbone, local area networks, data centers,supercomputing, to high-definition video. Due to superior bandwidth andlow loss, optical fibers are the medium of choice for transportinghigh-speed binary data.

Further limitations and disadvantages of conventional and traditionalapproaches will become apparent to one of skill in the art, throughcomparison of such systems with the present invention as set forth inthe remainder of the present application with reference to the drawings.

BRIEF SUMMARY OF THE INVENTION

A system and/or method for integrated power combiners, substantially asshown in and/or described in connection with at least one of thefigures, as set forth more completely in the claims.

Various advantages, aspects and novel features of the present invention,as well as details of an illustrated embodiment thereof, will be morefully understood from the following description and drawings.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1A is a block diagram of a photonically enabled CMOS chipcomprising integrated power combiners, in accordance with an embodimentof the invention.

FIG. 1B is a diagram illustrating an exemplary CMOS chip, in accordancewith an embodiment of the invention.

FIG. 1C is a diagram illustrating an exemplary CMOS chip coupled to anoptical fiber cable, in accordance with an embodiment of the invention.

FIG. 2 is a block diagram of an exemplary integrated transceiver, inaccordance with an embodiment of the invention.

FIG. 3 is a block diagram of an exemplary integrated transceiver with apolarization splitting function, in accordance with an embodiment of theinvention.

FIG. 4 is a block diagram of exemplary integrated transceiver withduplicate signal processors, in accordance with an embodiment of theinvention.

FIG. 5 is a block diagram of an exemplary optical power combiner, inaccordance with an embodiment of the invention.

FIG. 6 is a block diagram of an exemplary optical power combiner withphase modulators in each waveguide stage, in accordance with anembodiment of the invention.

FIG. 7 is a block diagram of an exemplary optical power combiner withphase modulators and power detection in each waveguide stage, inaccordance with an embodiment of the invention.

FIG. 8 is a block diagram of an exemplary power equalizer, in accordancewith an embodiment of the invention.

FIG. 9 is a block diagram of a polarization-insensitive combiner, inaccordance with an embodiment of the invention.

FIG. 10 is a block diagram of a polarization controller, in accordancewith an embodiment of the invention.

FIG. 11 is a block diagram of a polarization-insensitive splitter, inaccordance with an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Certain aspects of the invention may be found in a system for integratedpower combiners. Exemplary aspects of the invention may comprise a chipcomprising an optical power combiner in a photonic circuit, where theoptical power combiner comprises input optical waveguides, opticalcouplers, and output optical waveguides. Optical signals may be receivedin each of the input optical waveguides and phase-modulated to configurea phase offset between signals received at a first optical coupler,wherein the first optical coupler may generate output signals withsubstantially equal optical powers. One or both output signals of thefirst optical coupler may be phase-modulated to configure a phase offsetbetween signals received at a second optical coupler. The second opticalcoupler generates an output signal in a first of the output opticalwaveguides having an optical power of essentially zero and an outputsignal in a second of the output optical waveguides having a maximizedoptical power. The optical couplers may comprise, for example,directional couplers, and the chip may comprise, for example, a CMOSchip. Optical signals received by the input optical waveguides may begenerated utilizing a polarization-splitting grating coupler, whereinthe polarization splitting grating coupler enablespolarization-insensitive combining of optical signals utilizing theoptical power combiner. Optical power in waveguides coupling the opticalcouplers may be monitored using optical detectors. The monitoring ofoptical power may be used to determine a desired phase offset betweenthe signals received at the first optical coupler, and optical signalsmay be communicated to the optical detectors utilizing optical taps inthe coupling waveguides.

FIG. 1A is a block diagram of a photonically enabled CMOS chipcomprising integrated power combiners, in accordance with an embodimentof the invention. Referring to FIG. 1A, there is shown optoelectronicdevices on a CMOS chip 130 comprising optical modulators 105A-105D,photodiodes 111A-111D, monitor photodiodes 113A-113H, and opticaldevices comprising taps 103A-103K, optical terminations 115A-115D, andgrating couplers 117A-117H. There are also shown electrical devices andcircuits comprising amplifiers 107A-107D, analog and digital controlcircuits 109, and control sections 112A-112D. The amplifiers 107A-107Dmay comprise transimpedance and limiting amplifiers (TIA/LAs), forexample.

Optical signals are communicated between optical and optoelectronicdevices via optical waveguides 110 fabricated in the CMOS chip 130.Single-mode or multi-mode waveguides may be used in photonic integratedcircuits. Single-mode operation enables direct connection to opticalsignal processing and networking elements. The term “single-mode” may beused for waveguides that support a single mode for each of the twopolarizations, transverse-electric (TE) and transverse-magnetic (TM), orfor waveguides that are truly single mode and only support one modewhose polarization is TE, which comprises an electric field parallel tothe substrate supporting the waveguides. Two typical waveguidecross-sections that are utilized comprise strip waveguides and ribwaveguides. Strip waveguides typically comprise a rectangularcross-section, whereas rib waveguides comprise a rib section on top of awaveguide slab.

The optical modulators 105A-105D comprise Mach-Zehnder or ringmodulators, for example, and enable the modulation of thecontinuous-wave (CW) laser input signal. The optical modulators105A-105D comprise high-speed and low-speed phase modulation sectionsand are controlled by the control sections 112A-112D. The high-speedphase modulation section of the optical modulators 105A-105D maymodulate a CW light source signal with a data signal. The low-speedphase modulation section of the optical modulators 105A-105D maycompensate for slowly varying phase factors such as those induced bymismatch between the waveguides, waveguide temperature, or waveguidestress and is referred to as the passive phase, or the passive biasingof the MZI.

The phase modulators may have a dual role: to compensate for the passivebiasing of the MZI and to apply the additional phase modulation used tomodulate the light intensity at the output port of the MZI according toa data stream. The former phase tuning and the latter phase modulationmay be applied by separate, specialized devices, since the former is alow speed, slowly varying contribution, while the latter is typically ahigh speed signal. These devices are then respectively referred to asthe LSPM and the HSPM. Examples for LSPM are thermal phase modulators(TPM), where a waveguide portion is locally heated up to modify theindex of refraction of its constituting materials, or forward biased PINjunction phase modulators (PINPM) where current injection into the PINjunction modifies the carrier density, and thus the index of refractionof the semiconductor material. An example of an HSPM is a reversedbiased PIN junction, where the index of refraction is also modulated viathe carrier density, but which allows much faster operation, albeit at alower phase modulation efficiency per waveguide length.

The outputs of the modulators 105A-105D may be optically coupled via thewaveguides 110 to the grating couplers 117E-117H. The taps 103D-103Kcomprise four-port optical couplers, for example, and are utilized tosample the optical signals generated by the optical modulators105A-105D, with the sampled signals being measured by the monitorphotodiodes 113A-113H. The unused branches of the taps 103D-103K areterminated by optical terminations 115A-115D to avoid back reflectionsof unwanted signals.

The grating couplers 117A-117H comprise optical gratings that enablecoupling of light into and out of the CMOS chip 130. The gratingcouplers 117A-117D may be utilized to couple light received from opticalfibers into the CMOS chip 130, and the grating couplers 117E-117H may beutilized to couple light from the CMOS chip 130 into optical fibers. Thegrating couplers 117A-117H may comprise single polarization gratingcouplers (SPGC) and/or polarization splitting grating couplers (PSGC).In instances where a PSGC is utilized, two input, or output, waveguidesmay be utilized.

The optical fibers may be epoxied, for example, to the CMOS chip, andmay be aligned at an angle from normal to the surface of the CMOS chip130 to optimize coupling efficiency. In an embodiment of the invention,the optical fibers may comprise single-mode fiber (SMF) and/orpolarization-maintaining fiber (PMF).

In another exemplary embodiment, optical signals may be communicateddirectly into the surface of the CMOS chip 130 without optical fibers bydirecting a light source on an optical coupling device in the chip, suchas the light source interface 135 and/or the optical fiber interface139. This may be accomplished with directed laser sources and/or opticalsources on another chip flip-chip bonded to the CMOS chip 130.

The photodiodes 111A-111D may convert optical signals received from thegrating couplers 117A-117D into electrical signals that are communicatedto the amplifiers 107A-107D for processing. In another embodiment of theinvention, the photodiodes 111A-111D may comprise high-speedheterojunction phototransistors, for example, and may comprise germanium(Ge) in the collector and base regions for absorption in the 1.3-1.6 μmoptical wavelength range, and may be integrated on a CMOSsilicon-on-insulator (SOI) wafer.

The analog and digital control circuits 109 may control gain levels orother parameters in the operation of the amplifiers 107A-107D, which maythen communicate electrical signals off the CMOS chip 130. The controlsections 112A-112D comprise electronic circuitry that enable modulationof the CW laser signal received from the splitters 103A-103C. Theoptical modulators 105A-105D may require high-speed electrical signalsto modulate the refractive index in respective branches of aMach-Zehnder interferometer (MZI), for example. In an embodiment of theinvention, the control sections 112A-112D may include sink and/or sourcedriver electronics that may enable a bidirectional link utilizing asingle laser.

In operation, the CMOS chip 130 may be operable to transmit and/orreceive and process optical signals. The grating couplers 117A-117D maybe operable to receive optical signals from optical fibers coupled tothe chip 130 and may convert the optical mode of the fiber into the muchsmaller mode of a Si waveguide fabricated on the CMOS SOI wafer. Thegrating couplers 117A-117D may comprise a single-polarization or apolarization-splitting type: in the first case, only a specificpolarization is coupled to a single Si waveguide, while in the secondcase, two orthogonal polarizations are split into two output waveguides.

Integrated photonics platforms allow the full functionality of anoptical transceiver to be integrated on a single chip, the CMOS chip130, for example. A transceiver chip comprise opto-electronic circuitsthat create and process the optical/electrical signals on thetransmitter (Tx) and the receiver (Rx) sides, as well as opticalinterfaces that couple the optical signal to and from one or more fiber.The signal processing functionality may comprise modulating the opticalcarrier, detecting the optical signal, splitting or combining datastreams, and multiplexing or demultiplexing data on carriers withdifferent wavelengths.

The light source may be external to the chip or may be integrated withthe chip in a hybrid scheme. It is often advantageous to have anexternal continuous-wave (CW) light source, because this architectureallows heat sinking and temperature control of the source separatelyfrom the transceiver chip 130. An external light source may also beconnected to the transceiver chip 130 via a fiber interface.

An integrated transceiver may comprise at least three opticalinterfaces, including a transmitter input port to interface to the CWlight source, labeled as CW Laser In 101; a transmitter output port tointerface to the fiber carrying the optical signal, labeled OpticalSignals Out; and a receiver input port to interface to the fibercarrying the optical signal, labeled Optical Signals In.

Waveguide photodetectors may be incorporated in integrated opticsplatforms, where several components are integrated together on a singlereceiver chip, as illustrated in FIG. 1A. In this platform, lightcouplers, such as the optical couplers 117A-117D, couple the opticalsignal from the fiber into optical waveguides 110. The optical signalsubsequently enters the waveguide detectors 111A-111D, where it isconverted to an electrical signal. In some embodiments, the coupler maycomprise a grating coupler, in which case the fiber is oriented in anear normal configuration to the chip 130 surface.

In instances where the fiber medium carries the signal in a singleoptical mode, the receiver subsystem on the chip, comprising the lightcoupler, the waveguide, and the waveguide detector, may be designed tosupport a single mode. Because the single-mode fiber mode has twopolarization states, the term “single-mode waveguide” is used both forwaveguides that support a single mode for each of the two polarizations(TE and TM) or for waveguides that only support one mode whosepolarization is TE, with the electric field parallel to the substrate.

The fibers may be either single-mode fibers (SMFs),polarization-maintaining fibers (PMFs) or some other fiber type. Tofacilitate efficient optical signal processing, the waveguides carryingthe signal on the transceiver chip 130 may support one mode with asingle polarization. In contrast, the optical mode in SMFs has twoorthogonal polarizations. Since the CW light source has a well-definedpolarization, one option is to employ PMFs in order to retain a singlepolarization throughout the system. However, PMFs are costly and moredifficult to align accurately than SMFs. For this reason, SMFs may beused in an optical interconnect, thereby requiring input ports to acceptsignals in arbitrary polarizations. A polarization splitting gratingcoupler (PSGC) may be used to generate two optical modes from a receivedinput optical signal. In an exemplary embodiment of the invention, anoptical power combiner may be utilized to efficiently combine opticalsignals of unknown phase and intensity generated by the PSGC.

In an exemplary embodiment of the invention, the integrated power mayenable locating the CW laser source remotely, with the optical sourcesignal communicated to the CMOS chip 130 via optical fiber, as opposedto mounting a laser in the laser module 147 directly over a gratingcoupler.

FIG. 1B is a diagram illustrating an exemplary CMOS chip, in accordancewith an embodiment of the invention. Referring to FIG. 1B, there isshown the CMOS chip 130 comprising electronic devices/circuits 131,optical and optoelectronic devices 133, a light source interface 135,CMOS chip front surface 137, an optical fiber interface 139, and CMOSguard ring 141.

The light source interface 135 and the optical fiber interface 139comprise grating couplers, for example, that enable coupling of lightsignals via the CMOS chip surface 137, as opposed to the edges of thechip as with conventional edge-emitting devices. Coupling light signalsvia the CMOS chip surface 137 enables the use of the CMOS guard ring 141which protects the chip mechanically and prevents the entry ofcontaminants via the chip edge.

The electronic devices/circuits 131 comprise circuitry such as theamplifiers 107A-107D and the analog and digital control circuits 109described with respect to FIG. 1A, for example. The optical andoptoelectronic devices 133 comprise devices such as the taps 103A-103K,optical terminations 115A-115D, grating couplers 117A-117H, opticalmodulators 105A-105D, high-speed heterojunction photodiodes 111A-111D,and monitor photodiodes 113A-113H.

In an embodiment of the invention, the efficiency of receiver subsystemson the CMOS chip 130 may be increased by utilizing an optical powercombiner to efficiently combine optical signals of unknown phase andintensity in the photonic circuits in the CMOS chip 130. Otherembodiments of the invention comprise a power equalizer, apolarization-insensitive combiner, a polarization controller, and apolarization-insensitive splitter.

FIG. 1C is a diagram illustrating an exemplary CMOS chip coupled to anoptical fiber cable, in accordance with an embodiment of the invention.Referring to FIG. 1C, there is shown the CMOS chip 130 comprising theCMOS chip surface 137, and the CMOS guard ring 141. There is also showna fiber-to-chip coupler 143, an optical fiber cable 145, and an opticalsource assembly 147.

The CMOS chip 130 comprising the electronic devices/circuits 131, theoptical and optoelectronic devices 133, the light source interface 135,the CMOS chip surface 137, and the CMOS guard ring 141 may be asdescribed with respect to FIG. 1B.

In an embodiment of the invention, the optical fiber cable may beaffixed, via epoxy for example, to the CMOS chip surface 137. The fiberchip coupler 143 enables the physical coupling of the optical fibercable 145 to the CMOS chip 130.

In an embodiment of the invention, the efficiency of receiver subsystemson the CMOS chip 130 may be increased by utilizing an optical powercombiner to efficiently combine optical signals of unknown phase andintensity in the photonic circuits in the CMOS chip 130. Otherembodiments of the invention comprise a power equalizer, apolarization-insensitive combiner, a polarization controller, and apolarization-insensitive splitter.

In an exemplary embodiment of the invention, the integrated power mayenable locating the CW laser source remotely, with the optical sourcesignal communicated to the CMOS chip 130 via optical fiber, such as theoptical fiber cable 145, as opposed to mounting the laser directly overa grating coupler in the light source module 147.

FIG. 2 is a block diagram of an exemplary integrated transceiver, inaccordance with an embodiment of the invention. Referring to FIG. 2,there is shown an optical source 201, optical fibers 203A-203C, and atransceiver chip 210 comprising a Tx input coupler 205, opticalwaveguides 207A and 207B, a Tx processor 209, a Tx output coupler 211, aRx input coupler 213, and a Rx processor 215. The transceiver chip 210may, for example, be substantially similar to the CMOS chip 130.

The source 201 may comprise a continuous wave (CW) optical source, suchas a semiconductor laser, for example, that may provide an opticalsignal for the photonic circuitry in the transceiver chip 210. The Txinput coupler 205, the Tx output coupler 211, and the Rx input coupler213 may comprise grating couplers, for example, that may be operable tocouple light signals into and/or out of the transceiver chip to and/orfrom the optical fibers 203A-203C. The optical fibers 203A-203C maycomprise single-mode, polarization-maintaining, or other type of opticalfiber.

The Tx processor 209 may comprise a signal processor that may beoperable to modulate a CW optical signal utilizing an electrical signalto enable the communication of data from the transceiver chip 210 viathe Tx output coupler 211 and the fiber 203B. The Tx processor 209 maycomprise optical modulators and associated control circuitry, forexample, such as the optical modulators 105A-105D, the control sections112A-112D, and the control circuits 109. The Tx processor 209 may alsocomprise an optical wavelength multiplexer.

Similarly, the Rx processor 215 may be substantially similar to the Txprocessor 209, but operable to de-modulate optical signals received bythe transceiver chip 210 via the optical fiber 203C and the Rx inputcoupler 213 and extract electrical signals. The Rx processor 215 maycomprise one or more photodetectors to convert a received optical signalto an electrical signal. The Rx processor 215 may also comprise anoptical wavelength demultiplexer.

FIG. 3 is a block diagram of an exemplary integrated transceiver with apolarization splitting function, in accordance with an embodiment of theinvention. Referring to FIG. 3, there is shown an optical source 301,optical fibers 303A-303C, and a transceiver chip 310 comprising a Txinput coupler 305, optical waveguides 307A-307D, a Tx processor 309, aTx output coupler 311, a Rx input coupler 313, and a detector 317. Thetransceiver chip 310 may be substantially similar to the CMOS chip 130.

If the receiver signal processing function comprises simply thedetecting an optical signal, as illustrated in FIG. 3, an exemplaryembodiment comprises a polarization splitter function to the Rx inputport. In this embodiment, a light signal with an arbitrary polarizationstate in the optical fiber 303C is split into two separate opticalwaveguides 307C and 307D and is combined at the detector 317. The Rxinput coupler 313 may comprise a polarization-splitting grating coupler(PSGC). The intensity and phase of the light in each waveguide 307C and307D is thus a function of the input polarization state into thetransceiver chip 310 via the fiber 303C.

The Tx input coupler 305, the waveguides 307A-307D, the Tx processor309, and the Tx output coupler 311, for example, may be substantiallysimilar to the corresponding elements described with respect to FIG. 2.The detector 317 may, for example, be substantially similar to thephotodetectors 111A-111D, described with respect to FIG. 1A.

In instances where additional signal processing is required beforedetection, such as optical monitoring or demultiplexing, then eachsignal processing element would be duplicated for each optical path, theoptical waveguides 307C and 307D, as shown in FIG. 4.

FIG. 4 is a block diagram of exemplary integrated transceiver withduplicate signal processors, in accordance with an embodiment of theinvention. The transceiver chip 410 may, for example, be substantiallysimilar to the transceiver chip 310, described with respect to FIG. 3,but with the Rx processors 415A and 415B, which may, for example, besubstantially similar to the Rx processor 315, described with respect toFIG. 3. Thus, circuit complexity and power usage may be reduced bycombining the signals in each path before communicating them to a singleRx processor.

Similarly, at the transmitter input, if the fiber 403A connecting thelight source 401 to the Tx input coupler 405 is single-mode, then lightmay be split into two waveguides, such that the Tx signal processor 409would be duplicated, and the signals recombined before or at thetransmitter output. Thus, circuit complexity could be further reducedboth on the transmitter side and on the receiver side, with anopto-electronic circuit that combines the optical power from the outputsof the PSGCs efficiently.

FIG. 5 is a block diagram of an exemplary optical power combiner, inaccordance with an embodiment of the invention. Referring to FIG. 5,there is shown an optical power combiner 500 comprising input waveguides501A and 501B, phase modulators 503A and 503B, optical couplers 505A and505B, coupling waveguides 517, and output waveguides 507A and 507B. Theoptical couplers 505A and 505B may be directional couplers, ormulti-mode interference couplers, for example, and may exhibit a tappingratio of approximately 50%, for example. The directional couplers maycomprise a multi-stage directional coupler comprising a plurality ofdirectional couplers cascaded in series. The coupling waveguides 517 maybe operable to communicate optical signals between the couplers 505A and505B and the phase modulators 503A and 503B.

The phase and the intensity in the two output waveguides emanating froma PSGC is unknown since it depends on the polarization state in thefiber, so the light from the waveguides may not be combined passively,such as physically joining the waveguides side-by-side. This wouldviolate the physical principle known as the brightness theorem.Therefore, in an embodiment of the invention, the optical power combiner500 exhibits adaptive control to achieve an in-phase combination of thetwo input signals regardless of the polarization state of the incominglight in the input waveguides 501A and 501B.

In an embodiment of the invention, the optical power combiner 500combines light from the two input waveguides 501A and 501B into a singleoutput waveguide, given arbitrary intensity and amplitude in the twoinput waveguides. The output may be from either the output waveguide507A or 507B.

In an embodiment of the invention, the phase modulators may beadaptively adjusted to maximize the power in one of the outputwaveguides 507A and 507B for any input polarization state. Consequently,the signal is substantially extinguished in the alternate outputwaveguide. The amplitude of the light signal in input waveguides 501Aand 501B may be considered the two components of a vector (within aphase factor) as

$A\begin{pmatrix}{\cos\;\theta} \\{e^{i\;\phi}\sin\;\theta}\end{pmatrix}$

If the phase modulator 503A imparts a phase shift e^(−iϕ) in the inputwaveguide 501B relative to the input waveguide 501A, then before thecoupler 505A, the amplitude in the waveguides will be

$A\begin{pmatrix}{\cos\;\theta} \\{\sin\;\theta}\end{pmatrix}$

within a phase factor. After the coupler 505A, the amplitudes become

$\frac{A}{\sqrt{2}}\begin{pmatrix}e^{i\;\theta} \\{ie}^{{- i}\;\theta}\end{pmatrix}$

with equal power in both arms. If now the phase modulator 503B imparts aphase shift e^(2iθ) to the bottom waveguide, then before the coupler505B, the amplitudes will be

$\frac{A}{\sqrt{2}}\begin{pmatrix}1 \\i\end{pmatrix}$

within a phase factor. After the coupler 505B, we obtain

${iA}\begin{pmatrix}0 \\1\end{pmatrix}$

Since, depending on their design, the phase modulators 503A and 503Bnormally provide only positive or only negative phase shiftsefficiently, it may be desirable to insert additional phase modulatorsinto the waveguides in each stage.

FIG. 6 is a block diagram of an exemplary optical power combiner withphase modulators in each waveguide stage, in accordance with anembodiment of the invention. Referring to FIG. 6, there is shown anoptical power combiner 600 comprising input waveguides 601A and 601B,phase modulators 603A and 603B, optical couplers 605A and 605B, couplingwaveguides 617, and output waveguides 607A and 607B. The opticalcouplers 605A and 605B may be directional couplers, or multi-modeinterference couplers, for example, and may exhibit a tapping ratio ofapproximately 50%, for example. The optical power combiner 600 may, forexample, be substantially similar to the optical power combiner 500 butwith phase modulators in each waveguide stage. The coupling waveguides617 may be operable to communicate optical signals between the couplers605A and 605B and the phase modulators 603A-603D.

The optical power combiner 600 may be controlled, for instance, by usingpower monitors that tap some portion of the light off from bothwaveguides into detectors to monitor the signals, as illustrated furtherwith respect to FIG. 7. The phase modulators 603A and 603B may beconfigured so that the power detected in each path following the coupler605A is approximately equal. The phase modulators 603C and 603D may thenbe configured by maximizing the power in the desired output waveguide607A or 607B.

In another embodiment of the invention, the optical power combiner 600may comprise a plurality of stages, with each stage comprising pairs ofphase modulators/couplers. This may enable a larger capacity to correctfor unknown polarization fluctuations and uneven power splitting in theoptical couplers.

FIG. 7 is a block diagram of an exemplary optical power combiner withphase modulators and power detection in each waveguide stage, inaccordance with an embodiment of the invention. Referring to FIG. 7,there is shown an optical power combiner 700 comprising input waveguides701A and 701B, phase modulators 703A and 703B, optical couplers 705A and705B, output waveguides 707A and 707B, taps 709A and 709B, couplingwaveguides 717, and power detectors 711A and 711B. The optical couplers705A and 705B may be directional couplers, or multi-mode interferencecouplers, for example, and may exhibit a tapping ratio of approximately50%, for example. The optical power combiner 700 may, for example, besubstantially similar than the optical power combiner 600 but with phasemodulators in each waveguide stage. The coupling waveguides 717 may beoperable to communicate optical signals between the couplers 705A and705B, the phase modulators 703A-703D, the taps 709A and 709B, and thedetectors 711A and 711B.

The taps 709A and 709B may, for example, be substantially similar to thetaps 103A-103K described with respect to FIG. 1A, and may be operable totap optical power from the optical signals received from the coupler705A such that a measurement of the optical power may be measured andstill allow most of the optical signal to pass to the phase modulators703C and 703D. The power detectors 711A and 171B may comprisephotodetectors, for example, that may be operable to detect themagnitude of optical signals received from the taps 709A and 709B.

The optical power combiner 700 may be controlled by using the taps 709Aand 709B to tap a portion of the light off from both waveguides into thedetectors 711A and 711B to monitor the signals. The phase modulators703A and 703B may be configured so that the power detected in each pathfollowing the coupler 705A is approximately equal. The phase modulators703C and 703D may then be configured by maximizing the power in thedesired output waveguide 707A or 707B.

The combiner 700 may be part of a larger subsystem that also includesthe control electronics used for monitoring the tapped signal andcontrolling the amount of phase shift in each phase modulator. Thecontrol electronics may be either external to the transceiver chip orintegrated monolithically on the chip.

FIG. 8 is a block diagram of an exemplary power equalizer, in accordancewith an embodiment of the invention. Referring to FIG. 8, there is shownan optical power equalizer 800 comprising input waveguides 801A and801B, a phase modulator 803, an optical coupler 805, and outputwaveguides 807A and 807B.

In certain applications, it is beneficial to distribute light equallybetween two waveguides, given a power imbalance between the two. Thepower equalizer 800 is substantially the first stage of an exemplarypower combiner device, such as described with respect to FIG. 5, forexample, and comprises two input waveguides 801A and 801B, a phasemodulator 803, a coupler 805, and two output waveguides 807A and 807B.

In an embodiment of the invention, if the amplitudes of the light signalin waveguides 801A and 801B are written as

$A\begin{pmatrix}{\cos\;\theta} \\{e^{i\;\phi}\sin\;\theta}\end{pmatrix}$

then configuring the phase modulator 803 to impart a phase shift e^(−iϕ)in the input waveguide 801B relative to the waveguide 801A, theamplitude in the waveguides before the coupler 805 will be

$A\begin{pmatrix}{\cos\;\theta} \\{\sin\;\theta}\end{pmatrix}$

within a phase factor. After the coupler 805, the amplitudes become

$\frac{A}{\sqrt{2}}\begin{pmatrix}e^{i\;\theta} \\{ie}^{{- i}\;\theta}\end{pmatrix}$

Writing the amplitudes in terms of optical power,

$\frac{A^{2}}{2}\begin{pmatrix}1 \\1\end{pmatrix}$

that is, the powers in the output waveguides 807A and 807B are thusequal. As in the case of the power combiners 600 and 700, the opticalpower equalizer 800 may be augmented with an additional phase modulatorin the alternate input waveguide, a control system with taps andmonitors, and control electronics.

FIG. 9 is a block diagram of a polarization-insensitive combiner, inaccordance with an embodiment of the invention. Referring to FIG. 9,there is shown a polarization-insensitive combiner 900 comprising aninput fiber 901, a polarization-splitting grating coupler 915, phasemodulators 903A and 903B, optical couplers 905A and 905B, couplingwaveguides 917, and output waveguides 907A and 907B. The couplingwaveguides 917 may be operable to communicate optical signals betweenthe couplers 905A and 905B, the polarization splitting grating coupler915, and the phase modulators 903A and 903B.

In an exemplary embodiment of the invention, thepolarization-insensitive combiner 900 combines light from an arbitrarypolarization state in the fiber 901 into a single waveguide on thetransceiver chip, either output waveguide 907A and 907B depending on thecontrol of the phase modulators 903A and 903B, thereby reducing thecomplexity of other opto-electronic circuits on the chip.

The polarization-splitting grating coupler 915 accepts light from theinput fiber 901. Light with an arbitrary polarization is redirected intothe two output waveguides of the polarization-splitting grating coupler915, where the two signals can have an arbitrary phase and amplituderelationship. Using the combiner described with respect to FIG. 5following the polarization-splitting grating coupler 915, the power iscombined into a single output waveguide, either the waveguide 907A or907B. The polarization-splitting grating coupler 915 may be replacedwith any device having the functionality of a polarization splitter.

Controlling the phase modulators 903A and 903B may be achieved bymaximizing the signal in the output waveguide, 907A or 907B, orminimizing the signal in the alternate waveguide. As in the case of thepower combiner, the polarization-insensitive combiner 900 may becombined with an additional phase modulator in the alternate waveguides,a control system with taps and monitors, and control electronics.Furthermore, utilizing the polarization-insensitive combiner 900 at thetransmitter input port allows connecting the CW light source to thetransceiver chip using a single-mode fiber instead of apolarization-maintaining fiber, and on the receiver side, thepolarization-insensitive combiner 900 used as the input port obviatesthe need for duplicating the signal processing circuits on the receiver.

FIG. 10 is a block diagram of a polarization controller, in accordancewith an embodiment of the invention. Referring to FIG. 10, there isshown a polarization controller 1000 comprising an input fiber 1001, apolarization-splitting grating coupler 1015, phase modulators 1003A and1003B, optical couplers 1005A and 1005B, coupling waveguides 1017, andan output waveguide 1007. The coupling waveguides 1017 may be operableto communicate optical signals between the couplers 1005A and 1005B, thepolarization splitting grating coupler 1015, and the phase modulators1003A and 1003B.

The polarization controller 1000 may substantially comprise thepolarization-insensitive combiner 900 operating in reverse, such that itmay be used to launch light into a fiber or waveguide in any desiredpolarization state. The input waveguide 1007 receives the optical signalcoming from the rest of the opto-electronic circuit on the chip. Byadjusting the two phase modulators 1003A and 1003B, an arbitrarypolarization state may be generated in the output fiber 1001. As in thecase of the power combiners, the polarization controller 1000 may becombined with an additional phase modulator in the alternate waveguides,a control system with taps and monitors, and control electronics.

In another embodiment of the invention, the optical polarizationcontroller 1000 may comprise a plurality of stages, with each stagecomprising pairs of phase modulators/couplers. This may enable a largercapacity to correct for unknown polarization fluctuations and unevenpower splitting in the optical couplers.

FIG. 11 is a block diagram of a polarization-insensitive splitter, inaccordance with an embodiment of the invention. Referring to FIG. 11,there is shown a polarization controller 1100 comprising an input fiber1101, a polarization-splitting grating coupler 1115, a phase modulator1103, an optical coupler 1105, coupling waveguides 1117, and outputwaveguides 1107A and 1107B. The coupling waveguides 1117 may be operableto communicate optical signals between the coupler 1105, thepolarization splitting grating coupler 1115, and the phase modulator1103.

In multi-channel parallel transceiver architectures, power from thelight source is typically split between several channels. For atwo-channel system, the CW light coupled onto the chip is split evenlybetween the two channels before it enters the modulators. In anexemplary embodiment of the invention, the first stage of thepolarization-insensitive combiner 1100 may be utilized to achieve thisfunctionality.

As described with respect to the power combiners, the phase modulator803 may be adjusted so that the powers in the output waveguides 1107Aand 1107B are substantially equal for an arbitrary input polarization oflight in the fiber. In addition, as in the case of the power combiner,the polarization-insensitive combiner 1100 may be combined with anadditional phase modulator in the alternate waveguide, a control systemwith taps and monitors, and control electronics.

In another embodiment of the invention, the polarization-insensitivecombiner 1100 may also be used in a transmitter with more than twochannels. For instance, in a four-channel device, the polarizationinsensitive combiner and splitter may be followed by a passive splitterto further subdivide the incoming CW light into four, with each pair ofoutputs being controlled to output equal powers via phase modulationadjustments. And in yet another embodiment of the invention, thepolarization-insensitive combiner 1100 may also be used in the receiverof quadrature demodulation systems where light is split evenly betweentwo waveguides after it is received from a fiber.

In an embodiment of the invention, a method and system are disclosed fora chip 130, 210, 310, 410 comprising an optical power combiner 500, 600,700 in a photonic circuit 133, the optical power combiner 500, 600, 700comprising input optical waveguides 501A, 501B, 601A, 601B, 701A, 701B,optical couplers 505A, 505B, 603A-603D, 705A, 705B, and output opticalwaveguides 507A, 507B, 607A, 607B, 707A, 707B. Optical signals may bereceived in each of the input optical waveguides 501A, 501B, 601A, 601B,701A, 701B and phase-modulated to configure a phase offset betweensignals received at a first optical coupler 505A, 605A, 705A, where thefirst optical coupler 505A, 605A, 705A, may generate output signalshaving substantially equal optical powers. One or both output signals ofthe first optical coupler 505A, 605A, 705A may be phase-modulated toconfigure a phase offset between signals received at a second opticalcoupler 505B, 605B, 705B, where the second optical coupler 505B, 605B,705B generates an output signal in a first of the output opticalwaveguides 507A, 507B, 607A, 607B, 707A, 707B having an optical power ofessentially zero and an output signal in a second of the output opticalwaveguides 507A, 507B, 607A, 607B, 707A, 707B having a maximized opticalpower. The optical couplers 505A, 505B, 605A, 605B, 705A, 705B maycomprise grating couplers, for example, and the chip may comprise, forexample, a CMOS chip 130. Optical signals received by the input opticalwaveguides 501A, 501B, 601A, 601B, 701A, 701B may be generated utilizinga polarization-splitting grating coupler 313, 413, 915, 1015, 1115,where the polarization splitting grating coupler enablespolarization-insensitive combining of optical signals utilizing theoptical power combiner 500, 600. Optical power in waveguides 517, 617coupling the optical couplers 505A, 505B, 605A, 605B, 705A, 705B may bemonitored using optical detectors 711A, 711B. The monitoring of opticalpower may be used to determine a desired phase offset between thesignals received at the first optical coupler 505A, 605A, 705A, andoptical signals may be communicated to the optical detectors 711A, 711Butilizing optical taps 709A, 709B in the coupling waveguides 517, 617,717.

While the invention has been described with reference to certainembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted withoutdeparting from the scope of the present invention. In addition, manymodifications may be made to adapt a particular situation or material tothe teachings of the present invention without departing from its scope.Therefore, it is intended that the present invention not be limited tothe particular embodiments disclosed, but that the present inventionwill include all embodiments falling within the scope of the appendedclaims.

What is claimed is:
 1. A system comprising: a combiner operable to:generate a first optical signal and a second optical signal from agrating coupler that receives two or more optical signals of arbitrarypolarization; phase modulate one or both of the first optical signal andthe second optical signal to configure a first phase offset between thefirst optical signal and the second optical signal; communicate thefirst optical signal and the second optical signal with the first phaseoffset to a first optical coupler; phase modulate one or both of a thirdoptical signal and a fourth optical signal output from the first opticalcoupler to configure a second phase offset between the third opticalsignal and the fourth optical signal; and after phase modulation of oneor both the third optical signal and the fourth optical signal, combinethe third optical signal and the fourth optical signal using a secondoptical coupler to launch a first output signal, having an optical powerof essentially zero, into a first output waveguide of the second opticalcoupler and to launch a second output signal, having a maximized opticalpower, into a second output waveguide of the second optical coupler. 2.The system according to claim 1, wherein one or both of the firstoptical coupler and the second optical coupler comprise a directionalcoupler.
 3. The system according to claim 1, wherein one or both of thefirst optical coupler and the second optical coupler comprise amulti-mode interference coupler.
 4. The system according to claim 1,wherein one or both of the first optical coupler and the second opticalcoupler comprise a multi-stage cascaded directional coupler.
 5. Thesystem according to claim 1, wherein the combiner is integrated on aCMOS chip.
 6. The system according to claim 1, wherein optical detectorsare operable to monitor optical power in waveguides coupling the firstoptical coupler and the second optical coupler, wherein monitoring ofoptical power is used to determine a desired phase offset betweensignals received at one or both of the first optical coupler and thesecond optical coupler.
 7. The system according to claim 6, whereinoptical taps in the waveguides communicate optical signals to theoptical detectors.
 8. The system according to claim 1, wherein thecombiner comprises a plurality of stages.
 9. A method comprising:outputting from a grating coupler that receives an input signalcomprising two or more optical signals of arbitrary polarization, afirst optical signal and a second optical signal; phase modulating oneor both of the first and second optical signals to configure a firstphase offset between the first and second output signals; communicatingthe first and second optical signals with the first phase offset to afirst optical coupler; phase modulating one or both of a third opticalsignal and a fourth optical signal output from the first optical couplerto configure a second phase offset between the third and fourth opticalsignals; launching a first output signal, having an optical power ofessentially zero, into a first output waveguide of a second opticalcoupler; and launching a second output signal, having a maximizedoptical power, into a second output waveguide of the second opticalcoupler.
 10. The method according to claim 9, wherein one or both of thefirst optical coupler and the second optical coupler comprise adirectional coupler.
 11. The method according to claim 9, wherein one orboth of the first optical coupler and the second optical couplercomprise a multi-mode interference coupler.
 12. The method according toclaim 9, wherein one or both of the first optical coupler and the secondoptical coupler comprise a multi-stage cascaded directional coupler. 13.The method according to claim 9, wherein the method is performed in acombiner integrated on a CMOS chip.
 14. The method according to claim13, wherein the combiner comprises a plurality of stages.
 15. The methodaccording to claim 13, wherein the combiner comprises a single stage.16. The method according to claim 9, comprising monitoring optical powerin waveguides coupling the first optical coupler and the second opticalcoupler using optical detectors, wherein monitoring of optical power isused to determine a desired phase offset between signals received at oneor both of the first optical coupler and the second optical coupler. 17.A combiner operable to: receive, in a first optical coupler, a firstoptical signal and a second optical signal having arbitrarypolarization; generate a first phase offset between the first and secondoptical signals to output a third optical signal and a fourth opticalsignal; generate a second phase offset between the third and fourthoptical signals to output a fifth optical signal and a sixth opticalsignal; combine, in a second optical coupler, the fifth optical signaland the sixth optical signal to produce a combined optical signal,having essentially all optical power from the fifth optical signal andthe sixth optical signal, and a cancelled optical signal, monitor, viaoptical detectors, optical power in waveguides coupling the firstoptical coupler and the second optical coupler to determine a desiredphase offset between two optical signals received at one or both of thefirst optical coupler and the second optical coupler; and output thecombined output signal having a combined optical power of the fifth andsixth optical signals.
 18. The combiner of claim 17, wherein the secondoptical coupler includes a first output waveguide on which the combinedoptical signal is carried and a second output waveguide on which thecancelled optical signal is carried.